1. Field of the Invention
The present invention relates generally to a method and apparatus for the determination of the nature of the earth formations by neutron well logging. More specifically the present invention concerns the use of dual spaced detectors in a logging instrument in association with a third detector in the logging instrument and comparative data processing for detecting a ratio derived porosity measurement for the investigation of porous reservoir rock and coal bed formations. Even more specifically, the present invention concerns the use of a logging instrument having two substantially identical detectors and a differing third detector that are arranged to facilitate running of the logging instrument within well casing for optimized logging speed and efficiency and for achieving efficient and accurate formation logging. The logging instrument employs neutron-neutron sensors and a neutron-gamma capture sensor and a signal processing system that facilitates detection of signals for a ratio derived porosity measurement and capture gamma rays of various energies for earth formation evaluation.
2. Description of the Prior Art
Neutron well logging techniques and methods have been used to analyze earth formations along the traverse of a borehole for more than 30 years. One such accepted technique and method in accordance with prior art within the well logging industry to minimize the effects of formation capture properties resulting from salinity and shaliness as well as some borehole effects on the detection of thermal neutrons is by the use of two similar type detectors spaced differently from the neutron source. The signals from both detectors are combined to establish a ratio of detector count rates that is related to the porosity of the adjacent formation. While analyzing the dual detector signals could be used as a pore volume gas indicator, formation matrix change could be misleading. Also, differentiating between the oil and water phase of the pore volume has been questionable at best.
Another technique and method used to analyze earth formations along the traverse of a borehole is embodied in U.S. Pat. No. 3,772,513 of Hall, Jr. et al. This technique and method is used to determine the presence of hydrogen and chlorine (salinity) in a porous earth formation while compensating for shaliness and some borehole components. This in turn allows for water saturation determination and hydrocarbon identification. The device uses a rare earth sleeve or shield with a high capture cross-section and subsequent plurality emitting capture gamma radiation, surrounding a scintillation detector, to moderate neutron activity in the borehole and near-bore adjacent formations in order to achieve a balanced or compensated detector response from the formation capture gamma-rays of hydrogen and chlorine. The moderated formation capture hydrogen gamma-ray response could also be used to determine porosity if certain properties of the formation and borehole components are known.
Thus formation logging tools having a plurality of substantially identical detectors having substantially identical shields for significant thermal neutron induced gamma radiation response have been employed to establish a dual hydrogen or formation reference signal and a dual formation reference plus chlorine signal for detecting the presence of the chlorine constituent of salt water in the formation. However, these shielded detectors, being Neutron-Gamma scintillation type, made for a pore comparative ratio derived porosity measurement. Hence, a formation logging tool for additionally providing a Neutron-Neutron type ratio derived porosity measurement and a rare earth shielded, Neutron-Gamma scintillation type, capture gamma spectral measurement for identification of porous rock and coal bed formations intersected by a borehole has not, to the knowledge of the inventor, been available in the prior art.
In most cases it has been the practice to run formation logging instruments within an unlined borehole and to cause the logging tool to intimately engage a portion of the borehole wall, such as is shown in U.S. Pat. Nos. 3,219,820 and 3,772,513. The logging instruments of these patents each employ a decentralizing bow spring which engages the borehole wall and forces the housing of the instrument into intimate engagement with the open hole wellbore opposite the bow spring. When running a logging instrument through an open wellbore, efficient movement of the instrument may be subject to the interference of uneven wall surfaces, such as when relatively unconsolidated formation material is washed away by drilling fluid or sloughs into the wellbore. Additionally, these adverse wellbore conditions can cause the tools to become stuck in the wellbore resulting in costly retrieval or “fishing” operations for the operator of the well. If known hydrocarbon productive earth formations are present traversing the wellbore, it is more conducive and cost effective for the operator to lower and cement steel casing in the wellbore to secure and insure the integrity of the wellbore before well logging operations being. Unfortunately, the presence of steel casing eliminates or precludes most well logging tools to effectively measure the parameters of the earth formations within their design capability. Thus it is desirable to provide a formation logging instrument having a known two substantially identical detector arrangement and also having a third dissimilar detector, together with a signal processing system, additionally providing an efficient ratio derived porosity measurement and water saturation measurement for identification of porous rock and coal bed formations intersected by a cased borehole.